This invention relates to improvements in Schottky barrier semiconductor devices. Such devices are generally well known, for example in the form of asymmetrically conductive diodes comprising a body of appropriately doped semiconductor such as gallium arsenide with a region of its surface in intimate contact with an electrode of metal having a suitable work function, such as nickel, molybdenum, tungsten, palladium, or gold. The electrode is usually in the form of a thin film, deposited on the semiconductor by conventional techniques.
All of the metals used heretofore as Schottky electrodes exhibit one or more undersirable characteristics that require special countermeasures, which are usually expensive and not always fully satisfactory, in fabrication of the devices. For example, nickel films are brittle and vulnerable to mechanical stress; tungsten tends to form conductive whiskers that short-circuit the edge of the barrier; gold, an otherwise nearly ideal material, duffuses into the semiconductor and destroys its characteristics, particularly at higher temperatures within the desired operating range of the device.
For several reasons, for example its good thermal and electrical conductivity, its adaptability to thermal compression bonding, and resistance to corrosion, gold is also a nearly ideal material for making contact between the Schottky electrode and the external circuit. However, it diffuses readily through some metals such as palladium that would otherwise be suitable as electrodes. The prior art solution to this problem has been to provide a diffusion shield between the Schottky electrode and the gold terminal, comprising a film of some metal that resists gold diffusion.
The metals suitable as diffusion shields have such temperature coefficients of expansion that they must be sandwiched between additional metal layers of intermediate temperature coefficient to prevent destruction of the electrodecontact structure due to normal temperature variations. The two or more additional metal layers required in prior art practice contribute substantially to the cost and difficulty of fabricating such devices.
Passivation of the semiconductor device in prior art practice usually involves a sequence of steps such as deposition of one or more layers of insulating film and selective etching to define the desired patterns. The periphery of the barrier formed by the metal-semiconductor junction is particularly vulnerable to ambient reagents such as oxygen, water and sodium ions, and requires special precautions, as described in U.S. Pat. No. 3,635,417, for example.